Laminated, seamless, cylindrical metal screen for vacuum perforation of thermoplastic film

ABSTRACT

A laminated, cylindrical, metal screen or molding element for vacuum perforation of plastic film or sheets, comprising two or more relatively thin cylindrical metal screens, each having a predetermined inside and outside diameter and each having a plurality of openings or holes therein of a predetermined size and geometrical shape, and said relatively thin screens stacked and bonded together, diametrically one inside the other thereby providing a screen of a desired thickness and a desired hole geometry wherein the holes in the screen have substantially straight walls perpendicular to the surface of the screen. 
     A method of producing a relatively thick cylindrical metal screen for vacuum perforation of plastic film or sheets wherein the holes or openings in the screen have substantially straight walls perpendicular to the surface of the screen, comprising stacking and bonding together two or more matched relatively thin metal screens diametrically one inside the other.

BACKGROUND OF THE INVENTION

The present invention is in the general field of perforated plastic filmand especially relates to vacuum perforating of plastic film. Theinvention particularly relates to metal screens or molding elements usedin the vacuum perforation of plastic film and to a method of fabricatingsuch screens.

Perforated plastic film has many useful applications. It is used ingardening and farming to prevent the growth of grass and weeds whilepermitting moisture to be transmitted through the film to the soilbeneath. Perforated film is also used for making disposable diapers, forexample, see U.S. Pat. No. 3,814,101.

One of the earlier methods for vacuum perforation of plastic film isdisclosed in U.S Pat. No. 3,054,148. The patentee describes a stationarydrum having a molding element or screen mounted around the outer surfaceof the drum and adapted to freely rotate thereon. A vacuum chamber isemployed beneath the screen to create a pressure differential betweenthe respective surfaces of the thermoplastic sheet to be perforated tocause the plasticized sheet to flow into openings provided in the screenand thereby cause a series of openings, holes or perforations to beformed in the plastic sheet or film.

A variety of methods and apparatuses including particular types ofperforating screens or rotatable molding elements have been developedover the years for particular perforation operations. Examples of theseare U.S. Pat. Nos. 4,155,693, 4,252,516, 3,709,647, 4,151,240, 4,319,868and 4,388,056. In U.S. Pat. No. 4,155,693, the screen is comprised of aseries of perforated metal strips preferably welded together to form acylinder. U.S. Pat. No. 4,252,516 provides a screen having a series ofhexagonal depressions with elliptical holes centered therein. U.S. Pat.No. 3,709,647 provides for a rotating vacuum-forming roll having acirculating cooling medium therein.

U.S. Pat. No. 4,151,240 provides a means for cooling the film after ithas been perforated and debossed. U.S. Pat. No. 4,319,868 sets forth anapparatus for making a thermoplastic film having raised bosses withperforated tips. A particularly constructed embossing roll for effectingthe desired film pattern is disclosed. U.S. Pat. No. 4,388,056 disclosesan apparatus for continuously forming an air-laid fibrous web havingoppositely phased, cylindrically undulating side edges and apredetermined basis weight distribution. An air-laying drum has ahoneycomb type annular-shape frame including circumferentially extendingribs and transverse plates. A stationary adjustable air flow modulatingmeans is disposed adjacent the radially inwardly disposed boundary of anarcuate portion of a circumferentially segmented annular-shape plenum,circumferentially spanning a plurality of plenum segments for adjustinga pressure drop across particular areas of the surface of the air-layingdrum.

Vacuum perforation of thin plastic films involves the extrusion ofmolten polymeric materials such as polyethylene and other plasticpolymers though a slot die. The hot melt web of film or plastic sheetexiting the die impinges on a rotating cylindrical screen which ismounted on a stationary vacuum drum or roll. The vacuum roll has anaxial slot and a set of seals extending longitudinally the length of itsoutside surface, beneath the area where the web of plastic impinges onthe screen or molding element. A high vacuum from inside the screen isdirected through the slot in the vacuum roll. The vacuum present withinthe slot forms or molds the plastic film or sheet to the screen andperforates it through the holes of the screen. At the same time, anairflow is produced which cools the film.

The most important component of the vacuum processing equipment is thecylindrical screen. This molding element defines aesthetic andmechanical properties of the film as well as the geometric pattern ofthe perforated film. In a preferred screen fabrication technique, thedesired screen pattern is nickel plated on a specially preparedcylindrical mandrel. A seamless cylindrical nickel screen of anypredetermined or desired pattern can be produced. Other metals, such ascopper may also be used.

Some film products require the use of relatively thick screens, i.e.,from 0.020 to 0.100 inches thick, and also require that the walls of thepatterned holes are straight and perpendicular to the screen surface.Present screen fabrication techniques as heretofore described are notcapable of producing a screen meeting these requirements. The patternedholes on screens produced by nickel plating a prepared cylindricalmandrel, even with the application of specialized plating and postetching techniques, take the shape of inverted, truncated, concavedcones. The thicker the screen, the more exaggerated the effect becomes.

It is therefore a principal object of the present invention to provide amethod of fabricating relatively thick, seamless, cylindrical metalscreens having patterned holes whose walls are substantially straightand perpendicular to the surface of the screen.

Another principal object of the invention is to provide a thick,seamless, cylindrical metal screen which has patterned holes whose wallsare substantially straight and perpendicular to the screen surface.

Other objects and advantages of the instant invention will become morereadily apparent from the drawings and a reading of the descriptionhereinafter.

SUMMARY OF THE INVENTION

A relatively thick, laminated, cylindrical metal screen or moldingelement for vacuum perforation of plastic film or sheets is providedwhich comprises two or more relatively thin, cylindrical metal screens,each of which has predetermined inside and outside diameters and aplurality of openings or holes therein of predetermined size andgeometrical shape, which screens have been stacked and bonded togetherdiametrically one inside the other, and having a desired thickness and adesired hole geometry wherein the holes have substantially straightwalls which are perpendicular to the surface of the screen.

A method is also provided for producing the screens wherein matched setsof relatively thin, seamless, cylindrical metal screens are overplatedwith a thin layer of a bonding metal within specified tolerances whereinthe inside diameter of the largest screen fits the outside diameter ofthe next largest screen. Each screen fits similarly inside the nextlargest screen and has a common reference mark on each end thereof foralignment. Two relatively thicker seamless metal sleeves of differentdiameters are prepared wherein the inside surface of the larger diametersleeve is chrome plated and the outside surface of the smaller diametersleeve is chrome plated. The inside diameter of the larger sleeve fitsthe outside diameter of the largest diameter matched screen withinspecified tolerances. The outside diameter of the smaller sleeve fitsthe inside diameter of the smallest diameter matched screen withinspecified tolerances. The entire inside surfaces and the matched screensare coated with an appropriate flux. The largest cylindrical sleeve iscradled and clamped so as to retain its cylindrical shape. The largestdiameter screen is malformed, slid inside the cradled sleeve and thenreformed into its cylindrical shape. The next largest screen is thenmalformed and slid inside the reformed screen. The reference marks ofthe two screens are aligned and the screens are then pinned or otherwisefastened together. The smaller screen is then reformed into itscylindrical shape. The remaining screens of the matched set aresimilarly assembled. The smaller diameter sleeve is installed similarlyto the matched screens. The final assembly of sleeves and screens isthen heated in an oven to a suitable bonding temperature, cooled to nearambient temperature and then removed from the oven. Thereafter, theoutside sleeve is carefully cut axially and removed. Any remaining fluxis removed with an appropriate agent and the inside sleeve is removedthereby leaving a finished laminated screen product of desiredspecifications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustration an apparatus and method forvacuum perforation of thermoplastic film;

FIG. 2 is a side elevational view of a sleeve mounted in a cradle with ascreen partially inserted therein;

FIG. 3 is an end view of the assembly of FIG. 2 with the wall thicknessof the screen and sleeve enlarged to show detail;

FIG. 4 is an end view similar to that of FIG. 3, but illustrates acomplete assembly of screens and sleeves;

FIG. 5 is a perspective view of a laminated screen of the invention;

FIG. 6 is an enlarged view of a segment of the screen of FIG. 5;

FIG. 7 is a sectional view of the screen segment of FIG. 6 taken alongline 7--7;

FIG. 8 is a sectional view of a screen segment similar to that of FIG. 7illustrating an alternate hole radial cross-section;

FIG. 9 is a sectional view of a screen segment similar to that of FIG. 7illustrating another alternate hole radial cross-section; and

FIG. 10 is a sectional view of a screen segment similar to that of FIG.7 illustrating still another alternate hole radial cross-section.

Referring now to FIG. 1 of the drawings, vacuum process equipment andmethod for perforating plastic film or sheet therewith are schematicallyillustrated. A plastic polymer such as polyethylene is heated to a meltand extruded from an extruder E through a die D where a film web 10 isformed. The web 10 is directed onto a rotating cylindrical screen 11having a desired pattern of holes which is turning clockwise asindicated by arrow A around a stationary vacuum roll or drum 12. Avacuum chamber 13 within the roll 12, longitudinal air slot 14 and seals15 are employed to create a pressure differential between the respectivesurfaces of the thermoplastic sheet or web 10 to cause the plasticizedsheet to flow into the holes in the screen in the direction indicated bythe arrow B and therefore perforate the web 10. The perforated film web10' travels onto a guide roll 16 and continues onto a wind-up roll 17.Additional guide rolls or tensioning rolls can be employed as desired aswell as various film treating equipment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Seamless cylindrical nickel screens produced by plating are made by twoprincipal methods. In one method, a mandrel is ground to a dimensioncorresponding to the desired dimension of the finished screen. Thedesired screen hole pattern is engraved on the mandrel either by spiralengraving or malett indexing techniques. A plating resist medium isapplied to the surface of the mandrel, filling the pattern produced bythe engraving. A finish grinding operation is employed to remove theplating resist medium from the land areas between the engraved pattern,so that only the land area will be subject to plating. Subsequent nickelplating of the mandrel forms the screen. The mandrel is cooled and thefinished screen is removed therefrom.

The other method of producing a seamless cylindrical nickel screen issimilar to the foregoing method, except that instead of engraving thepattern on the mandrel, a special photographic negative of the patternis created and wrapped around the mandrel. Exposing of the negativetransfers the pattern to the mandrel in the form of a plating resist,leaving only the land areas of the screen available for plating. Themandrel is then plated, forming the screen which is then removed fromthe mandrel to obtain the finished product.

Either of these screen manufacturing methods are satisfactory to producethin screens for lamination; however, the latter method is preferred.Using a photographic negative permits relative ease in transferring apattern from one diameter screen to an identical one on another diameterscreen by reduction of the photographic negative.

In preparing a matched set of screens for lamination, severalrequirements must be met. First, each successive screen of the set mustfit diametrically inside the next larger diameter screen, withsufficient space allowed for the metal or tin-lead alloy plating of eachscreen. Secondly, each screen of the set must be geometricallyidentical. This means that each screen has the same number of holes,both axially and circumferentially, and that the radial centerlines ofeach hole of any one screen match identically the radial centerlines ofthe corresponding holes in all other screens in the set. Finally, theindividual hole geometry of all the aligned holes in all the screensmust be nearly identical.

The inside diameter of each individual thin screen to be laminated maybe calculated by the following formula:

    Dn=D+2tp+(n-1) (2t.sub.L +4tp)

wherein

Dn=Inside diameter of each individual thin screen n;

D=Inside diameter of the finished laminated screen;

n=The number of individual thin screens, counting from inside out, n=1,2, 3, . . . N;

t_(L) =The individual thin screen thickness (assumed constant for eachscreen of the set); and,

tp=The thickness of the metal plating to be applied (assumed constantfor each screen of the set).

The inside diameter of the outer solid metal sleeve may be calculated bythe following formula:

    Do.s=D.sub.N +2t.sub.L +2tp+2tc

wherein

Do.s=Inside diameter of the outer sleeve;

DN=Inside diameter of the Nth thin screen; and,

tc=The thickness of the chrome plating to be applied.

The inside diameter of the inner solid metal sleeve may be calculated bythe following formula:

    DI.s=D-2ts-2tc

wherein

ts=The thickness of the solid metal sleeves (assumed to be same for eachsleeve).

Using the foregoing procedures and formulas, matched sets of seamless,cylindrical metal (nickel) screens may be produced as follows:

1. Grind a mandrel to a dimension corresponding to the finished sleeveinside diameter of Do.s, produce the outer cylindrical metal sleevethereon, and plate the sleeve to a thickness of ts.

2. Grind the same mandrel to a dimension corresponding to the finishedinside diameter of the Nth screen. Apply a desired screen pattern usingthe photo-negative resist method including a reference alignment mark ateach end of the screen. Plate the mandrel to a thickness of t_(L),forming the largest, "Nth", screen of the matched set.

3. Grind the same mandrel to a dimension corresponding to the finishedinside diameter of the (N-1)th screen. Reduce the photographic negativein the circumferential direction only to equal the circumference of thereground mandrel, and apply the pattern to the mandrel as before. Platethe mandrel to a thickness of t_(L) forming the next largest, (N-1)th,screen of the matched set. The photographic negative reduction andtransfer operation may be a computerized operation.

4. Repeat Step 3 for each of the remaining screens of the matched set.

5. Grind the same mandrel to a dimension corresponding to the finishedsleeve inside diameter of DI.s., produce the inner cylindrical sleevethereon, and plate the sleeve to a thickness of ts.

Since screens are generally prepared for particular film perforatingoperations, the use of the same mandrel is preferable. Should a numberof laminated screens of the same pattern be produced, a number ofmandrels may be used, as appropriate.

In the preferred form of the invention, a matched set of thin, seamless,cylindrical, nickel screens are prepared as set forth in detailhereinafter. The length and diameter of the screens is selected on thebasis of the type of vacuum perforating equipment and the number ofscreens to be laminated. The thickness of each screen is determined onthe basis of the type of hole wall shape desired. A screen thickness ofapproximately 0.005 inches provides hole walls that are substantiallystraight and perpendicular to the surface of the screen.

Each screen of the matched set is overplated with a thin layer of abonding material, preferably a tin-lead alloy. An alloy of 63 weightpercent tin and 37 percent lead is especially preferred. An alloy layerof about 0.00025 inches thick is most preferred. Each screen of thematched set is designed and produced so that after plating, the insidediameter of the largest screen fits, within a specified tolerance, theoutside diameter of the next largest screen. In turn, each screen of theset fits similarly inside the next largest screen. Each screen of theset is designed and produced with a common reference or aligning mark ateach end thereof. Alignment of the reference marks during assemblyproduces proper pattern hole alignment.

Two seamless cylindrical nickel sleeves of different diameters are alsorequired. The sleeves are about the same length of the thin screens. Thethickness of the sleeves may be varied, but a thickness of approximately0.010 inches is especially preferred. The inside surface of the largerdiameter sleeve, and the outside surface of the smaller diameter sleeveare chrome plated. A plating thickness of about 0.0005 inches is mostpreferred. The sleeves are designed and produced so that after chromeplating, the inside diameter of the larger sleeve fits, within aspecified tolerance, the outside diameter of the largest diameter thinscreen, after the screen has been plated. AIso, after chrome plating,the outside diameter of the smaller sleeve is designed and produced tofit the inside diameter of the smallest diameter thin screen of thematched set of screens, after the screen has been plated. The twosleeves are used as fixtures during the assembly and bonding operationsas explained hereinafter.

The tin-lead alloy plating applied to each screen provides the bondingmedium for lamination. The chrome plating applied to the sleeves acts asa resist to prevent the tin screens being laminated from bonding to thesleeves.

After the matched set of thin screens and the two sleeves have beenprepared in the manner described, the assembly and bonding operationscan be completed.

Referring now to FIGS. 2-4 of the drawings, the larger cylindricalsleeve 20 is mounted in a cradle 21 and restrained therein so as toretain its cylindrical shape by two stiff outside diameter ring clamps22 and 23. Any other suitable restraining or clamping means may be used.The largest diameter thin screen 30 of the matched set of screens ismalformed or otherwise distorted and slid or positioned inside thelarger diameter sleeve 20. The screen 30 is then reformed into itscylindrical shape.

The next largest screen 31 is similarly malformed and slid inside thelargest screen 30 of the assembly 40. Reference marks 32 on the ends ofthe two screens are aligned and the screens are pinned or otherwisefastened together.

If more than two thin screens are to be laminated or assembled, theremaining thin screens are similarly installed with the next largestthin screen being positioned inside the second thin screen 31 and so on,until each thin screen of the matched set of thin screens has beenassembled.

Once all the thin screens have been assembled, the smaller sleeve 24 ispositioned within the smallest diameter thin screen in the same manneras the various thin screens were installed. Prior to the installation ofthe smaller sleeve, the entire inside surface of the assembly isthoroughly coated with an appropriate flux, for example, a liquid solderflux having a pure resin base or a soldering paste containing zincchloride, both manufactured by CG Electronics. Any other suitable fluxmay be used.

Once the assembly 40 is completed, it is transferred to an oven anduniformly heated to a temperature of about 400° F.±10° F. or 390° F. to410° F. During the bonding cycle, the cylindrical nickel sleeves remainon the assembled screens. The sleeves act to uniformly restrain thescreen laminates while permitting growth and shrinkage during heatingand cooling cycles as they are preferably of the same material as thethin screens. The assembly is then allowed to cool in the oven to nearlyroom or ambient temperature. It is then removed from the oven for finaloperations.

After the assembly has been cooled and removed from the oven, theoutside sleeve is carefully axially cut and removed. Remaining fluxresidue is removed from the screen assembly with a suitable agent, forexample, by washing with a degreasing liquid followed by a neutralizingwash of a 2 percent solution of muriatic acid. Any other suitable fluxremoving agent may be used. The inside sleeve is then removed, leaving afinished laminated screen of a desired thickness.

Although other bonding mediums may be used, the specific one of 63%tin-37% lead, plated to a thickness of 0.00025 inches particularly meetsthe requirements of the fabricating system. Pure, plated nickelinitiates embrittlement at approximately 425° F. Any bonding systemselected must therefore cure or flow at a temperature below theembrittlement temperature of the nickel plating. The tin-lead alloypreferred is the eutectic formulation of the two elements. The alloymelts and solidifies at 361° F. The instant bonding system is unique, inthat, by plating each individual thin screen prior to assembly,difficult and messy applications of bonding mediums during assembly areavoided. The tin-lead alloy plating thickness of 0.00025 inches is theoptimum thickness to produce good bonding with minimum excess alloy.

Referring now to FIG. 5, a laminated screen 50 of the invention is seenwhich comprises four thin screens 51, 52, 53 and 54 which have beenstacked and bonded together in accordance with the foregoing procedure.

As seen in FIG. 6, the screen 50 has a pluarlity of openings or holes50a therein. The holes 50a have walls 50b which are substantiallystraight as best seen in FIG. 7. In the latter figure, it is readilyseen that the walls 50b are formed from the individual walls 51b, 52b,53b and 54b of each of the tin screens 51, 52, 53 and 54, respectively.For purposes of illustration, the truncated holes of the thin screenshave been exaggerated. The effect of the multiscreen lamination provideshole walls which are substantially straight.

While the invention is particularly directed to a method of producingrelatively thick seamless cylindrical metal screens, especially nickelscreens, with holes or openings which are essentially straight andperpendicular to the screen surface, the method can also be utilized toprovide thick screens with an expanded variety of hole radialcross-sections. It is only essential that the basic hole radialcenterlines of each screen laminate be the same.

Although for simplification, the invention is illustrated with screenshaving round holes, other hole configurations or patterns are suitable.Such holes may be oval, rectangular, pentagonal, hexagonal or any otherdesired geometric shape.

FIGS. 8-10 illustrate some of the screen arrangements that can beachieved by using thin screens of different hole sizes. It can readilybe appreciated that a wide variety of hole shapes and sizes can beobtained for providing a particular hole effect in a thermoplastic filmor sheet.

Although the invention is particularly suited for perforatingthermoplastic sheets or film made from polyolefins, especiallypolyethylene and polypropylene, it can be used with other types ofthermoplastic films as desired.

The foregoing disclosure and description of the invention isillustrative and explanatory thereof and various changes in theillustrated construction may be made within the scope of the appendedclaims without departing from the spirit of the invention.

What is claimed is:
 1. A laminated, seamless, cylindrical metal screenor molding element for vacuum perforation of plastic film or sheets,comprising two or more relatively thin geometrically identical,seamless, cylindrical metal screens, each of said relatively thinscreens being sufficiently thin so that it may be readily malformed andsubsequently reformed to its original cylindrical shape, each of saidrelatively thin screens having a plurality of openings or holes therein,said relatively thin screens stacked and bonded together diametricallyone inside the other to provide the laminated cylindrical metal screen,the holes of each of said thin screens being substantially aligned withthe holes of the other of said thin screens, the individual geometry ofthe aligned holes in said relatively thin screens being substantiallyidentical, the holes of said laminated, seamless, cylindrical metalscreen thereby having substantially straight walls perpendicular to thesurface of said laminated, seamless, cylindrical metal screen, and saidlaminated, seamless, cylindrical metal screen having a total screenthickness of about 0.01 to 0.1 inches.
 2. The laminated, cylindricalmetal screen of claim 1 wherein each of said relatively thin screens hasa thickness of about 0.005 inches.
 3. The laminated, cylindrical metalscreen of claim 1 wherein the metal is nickel.
 4. The laminated,cylindrical metal screen of claim 1 wherein the metal is copper.
 5. Thelaminated, cylindrical metal screen of claim 1 wherein each of therelatively thin screens is overplated with a tin-lead plating.
 6. Thelaminated, cylindrical metal screen of claim 5 wherein the tin-leadplating is about 63 weight percent tin and 37 weight percent lead. 7.The laminated, cylindrical metal screen of claim 1 wherein saidindividual geometry of said holes in each of said relatively thinscreens is a square.
 8. The laminated, cylindrical metal screen of claim1, wherein said individual geometry of said holes in each of saidrelatively thin screens is a pentagon.
 9. The laminated, cylindricalmetal screen of claim 1, wherein said individual geometry of said holesin each of said relatively thin screens is a hexagon.
 10. The laminated,cylindrical metal screen of claim 1, wherein there are about 100 to20,000 holes per square inch of screen surfaces.
 11. The laminated,cylindrical metal screen of claim 10 wherein each of said holes has adiameter of about 0.015 to 0.020 inches.
 12. A laminated, seamless,cylindrical metal screen or molding element for vacuum perforation ofplastic film or sheets, comprising two or more relatively thin, seamlesscylindrical metal screens, each of said relatively thin screens beingsufficiently thin so that it may be readily malformed and subsequentlyreformed to its original cylindrical shape, each of said relatively thinscreens having a plurality of openings or holes therein, said relativelythin screens being stacked and bonded together diametrically one insidethe other to provide the laminated, seamless, cylindrical metal screen,the holes of each of said thin screens being substantially aligned withthe holes of the other of said thin screens, each of the holes in one ofsaid thin screens having a radial centerline matching a radialcenterline in each of said holes of said other relatively thin screens,and the holes of at least one of said thin screens having a greaterradius than the radius of the holes of at least one of said other thinscreens, and said laminated, seamless, cylindrical metal screen having atotal screen thickness of about 0.01 to 0.1 inches.